A gas spring strut is essentially a sealed cylinder containing a near ideal gas (usually nitrogen under a high pressure, typically about 1000-2000 psi) with a piston in contact with the gas to force movement of a piston rod extending from one end of the cylinder. The use of gas struts as a spring element offers many advantages, but has a major drawback in that as the ambient temperature falls, the pressure of nitrogen, which follows the ideal gas law, drops by about 26% from +8.degree. C. to -40.degree. C., reducing the movement force of the piston rod by the same amount. Thus, gas spring struts have the disadvantage that the force with which they expand is a function of ambient temperature. If used as an automotive liftgate strut, the rate at which the strut will lift will slow as the temperature falls, and in very cold weather, the strut may not be able to lift the liftgate completely. Attempting to fix this problem by using higher force struts may only cause the liftgate to open too rapidly at high temperatures.
U.S. Pat. No. 4,613,115 proposes a gas spring strut that has two distinctly different high pressure gas volumes separated by the seal of the piston. One of the gases is considered primary and is constituted of nitrogen, the other is a secondary gas (such as freon) that changes pressure at a faster rate with temperature than does nitrogen. The secondary gas produces a force opposing the lifting force of the nitrogen so that the net lifting force is the difference in the two forces. The pressure of the secondary gas changes with temperature in such a way that the net lifting force can remain relatively constant; the secondary gas is selected in this patent to exert an opposing force that is the vapor pressure of the system; the secondary gas must be a gas which provides a continuous two-phase gas system in which the liquid and vapor phases are in equilibrium over the temperature range of -30.degree. C. to +80.degree. C. Patent '115 demonstrates only three gases that remain as a two-phase system within such temperature range, namely, ammonia, freon-12, and sulfur hexafluoride. Other gases suggested were not proven to be operable in such range (such as acetylene, ethane, propane, propadiene, perfluoride propane, dimethyl ether, n-butane, hydrogen bromide, and hydrogen iodide). It is noted that the patent concept has never been offered commercially because of the difficulty of assembling the different high pressure gases into their respective volume chambers while preventing the gases from mixing by bypassing the piston seal that separates the different volume chambers. Because of this, it is also noted that the patented concept had difficulty in achieving essentially constant net operating force for the piston rod with varying temperature.